In the electrophotographic process, a photoconductor is charged in the dark, then exposed to a light image of an original document, drawing, or picture to be copied. In the areas struck by light, the charge is wholly or partially neutralized, depending on the intensity of the light, thus forming a latent electrostatic image on the surface of the photoconductor. If the photoconductor is selenium, the latent image will have a positive electrostatic charge; if the photoconductor is cadmium sulphide, the latent image will have a negative electrostatic charge. The image is then developed by exposing it to charged particles of a toner.
In the processes of the prior art, the developed image has been transferred to a carrier sheet, which may be of any suitable sheet material such as paper, polyester, polyacetate, polycarbonate, or the like. The transfer is accomplished by placing the carrier sheet in contact with the developed electrostatic image and assisting transfer by subjecting the back of the carrier sheet to a potential of a polarity opposite to the charge of the toner particles forming the developed electrostatic image. This will attract the toner particles forming the image to the carrier sheet and effect a transfer of the developed image. If the image is formed of adhesive toner particles, the transfer may be by adhesion after contact, assisted by pressure applied to the rear of the carrier sheet by a roller. This roller may be made of conductive material and biased to a potential having a polarity opposite to the polarity of the charge of the toner particles forming the developed electrostatic image. My process will be described with special reference to a latent electrostatic image which has been developed by electrophoresis of charged toner particles suspended in a dielectric liquid carrier.
The transfer step of the prior art is usually accomplished as pointed out above. This requires contact of the carrier sheet with the freshly developed electrostatic image. In order to accomplish adequate transfer, the developed image must be in a moist condition. If it is too dry, there will be difficulty in transferring the image from the surface of the photoconductor to the carrier sheet. The carrier liquid is usually a non-toxic light paraffinic hydrocarbon, preferably one which has been isomerized so that it will have a very narrow boiling range. Since the freshly developed electrostatic image must be moist, toner is squashed during the transfer by contact with the carrier sheet. This reduces resolution. Since the carrier sheet is usually paper, it will be absorbent. This requires drying of the image, which results in evaporation of the carrier liquid in the circumambient atmosphere. The evaporation of any hydrocarbon into the atmosphere is considered a pollutant, and the amount of evaporation permitted is strictly controlled. This reduces the speed at which an electrophotographic copying machine can be operated. Furthermore, the non-toxic light paraffinic hydrocarbon carrier is expensive and the amount evaporated must be replaced. After the developed image is transferred to a carrier sheet, it will be strongly adhered to the carrier sheet by the polarity of the charge on the rear of the carrier sheet. The charge of the particles, however, is opposite to that of the charge of the latent electrostatic image. The arrangement is such that the paper tends to stick to the photoconductive surface. The greater the density of the developed image, the greater will be the tendency of the carrier sheet to stick to the photoconductive surface. This produces some difficulty in removing the carrier sheet bearing the developed image from the photoconductive surface. The usual carrier sheet is paper, and the repetitive contact of paper with the wet developed image leaves paper fibers on the photoconductive surface. Since all of the developed image is rarely transferred to the carrier sheet, the paper fibers contaminate the developing liquid. Since the contact with the paper squashes the moist developed image, not only is resolution reduced, but the gradation of density, or gray scale, is also reduced.